Laser-markable plastics

ABSTRACT

The present invention relates to laser-markable plastics, in particular thermoplastic polyurethanes, which are distinguished by the fact that they contain pigments having a coating of doped tin dioxide.

This is a divisional, of application Ser. No. 08/953,796 filed Oct. 17,1997; which is a continuation of application Ser. No. 08/732,448 filedNov. 4, 1996 now abandoned; which is a 371 of PCT/EP95/01672, filed May3, 1995.

The present invention relates to laser-markable plastics which aredistinguished by the fact that they contain pigments having a coating ofdoped tin dioxide.

The labelling of products is becoming of increasing importance invirtually all sectors of industry. Thus, for example, production dates,use-by dates, bar codes, company logos, serial numbers, etc., mustfrequently be applied. At present, these marks are predominantly madeusing conventional techniques such as printing, embossing, stamping andlabelling. However, the importance of non-contact, very rapid andflexible marking using lasers, in particular in the case of plastics, isincreasing. This technique makes it possible to apply graphicinscriptions, for example bar codes, at high speed even on a non-planarsurface. Since the inscription is in the plastic article itself, it isdurable and abrasion-resistant.

Many plastics, for example polyolefins and polystyrenes, have hithertoproven to be very difficult or even impossible to mark by means oflasers. A CO₂ laser which emits light in the infrared region at 10.6 μmproduces only a weak, virtually illegible mark in the case ofpolyolefins and polystyrenes, even at very high output. In the case ofthe elastomers polyurethane and polyether-esters, Nd-YAG lasers produceno interaction, and CO₂ lasers produce an engraving. The plastic mustnot fully reflect or transmit the laser light, since no interaction thenoccurs. Neither, however, can there be strong absorption, since in thiscase the plastic evaporates and only an engraving remains. Theabsorption of the laser beams and thus the interaction with the materialdepends on the chemical structure of the plastic and the laserwavelengths used. It is in many cases necessary to add appropriateadditives, for example absorbers, to make plastics laser-inscribable.

The laser labelling of plastics is increasingly being carried out usingNd-YAG lasers in addition to CO₂ lasers. The YAG lasers usually usedemit a pulsed energy beam having a characteristic wavelength of 1064 nmor 532 nm. The absorber material must exhibit pronounced absorption inthis specific NIR region in order to exhibit an adequate reaction duringthe rapid inscription operations.

DE-A 29 36 926 discloses that the inscription of a polymeric material bymeans of laser light can be achieved by admixing the plastic with afiller which discolours on exposure to energy radiation, such as carbonblack or graphite.

EP 0 400 305 A2 describes highly polymeric materials which can beinscribed by means of laser light and which contain copper(II) hydroxidephosphate or molybdenum(VI) oxide as discolouring additive.

A plastic moulding composition based on an organic thermoplastic polymerand containing a black pigment and which can be provided with charactersby exposure to laser radiation is disclosed in EP 0 522 370 A1.

However, all the fillers known from the prior art have the disadvantagethat they .durably colour the plastic to be inscribed and consequentlythe laser inscription, which is usually a dark script on a palerbackground, is then no longer sufficiently high in contrast.

The filler or the successful absorber should therefore have a very paleinherent colour or need only be employed in very small amounts. Thecontrasting agent antimony trioxide satisfies such criteria. U.S. Pat.No. 4,816,374 employs antimony trioxide for laser inscription inthermoplastic elastomers by means of Nd-YAG lasers. It is employed in aconcentration of from 3 to 8%, depending on the matrix material and thewriting speed of the laser. Laser marking is possible using cadmium andarsenic compounds, but such substances are no longer used owing to theirtoxicity.

The object of the present invention was therefore to find laser-markableplastics which enable high-contrast marking on exposure to laser lightand contain only small amounts of heavy metals.

Surprisingly, it has been found that thermoplastics containing pigmentswhich have a coating of, for example, antimony-doped tin dioxide enablehigh-contrast marking with sharp edges.

The invention therefore relates to laser-markable plastics which arecharacterized in that thermoplastics contain pigments having a doped tindioxide coating.

The addition of the pigments in concentrations of from 0.1 to 4% byweight, preferably from 0.5 to 2.5% by weight, in particular from 0.3 to2% by weight, based on the plastic system, achieves a contrast on lasermarking which corresponds to or is even superior to that of a plasticcontaining significantly more antimony trioxide in terms ofconcentration. However, the concentration of the pigments in the plasticdepends on the plastic system employed. The small proportion of pigmentchanges the plastic system insignificantly and does not affect itsprocessing properties.

The tin dioxide coating of the pigments is preferably doped withantimony, arsenic, bismuth, copper, gallium or germanium, in particularwith antimony, or the corresponding oxides. The doping can amount to0.5-50% by weight, preferably 0.5-40% by weight, in particular 0.5-20%by weight, based on the tin dioxide. The tin dioxide coating may be aconductive or non conductive coating.

Transparent thermoplastics doped with such pigments in a purecolouration exhibit slight metallic shimmering, but retain theirtransparency. The addition of from 0.2 to 10% by weight, preferably from0.5 to 3% by weight, of opaque pigments, for example titanium dioxide,can, if required, completely hide this metallic sheen, in particular inthe case of thermoplastic polyurethane. Furthermore, coloured pigmentswhich allow colour variations of all types and simultaneously ensureretention of the laser marking, can be added to the plastics.

The pigments suitable for the marking and their preparation processesare described, for example, in DE-A 38 42 330 and EP 0 139 557. Thepigments are preferably based on platelet-shaped, preferably transparentor semi-transparent substrates of, for example, phyllosilicates, suchas, for example, mica, talc, kaolin, glass, SiO₂ flakes, synthetic orceramic flakes or synthetic support-free platelets. Also suitable aremetal platelets, for example aluminium platelets, or platelet-shapedmetal oxides, for example iron oxide or bismuth oxychloride. Theparticularly preferred substrate comprises mica flakes coated with oneor more metal oxides. The metal oxides used here are either colourless,high-refraction metal oxides, such as, in particular, titanium dioxideand/or zirconium dioxide, or coloured metal oxides, for example chromiumoxide, nickel oxide, copper oxide, cobalt oxide and in particular ironoxides.

The tin dioxide coating is applied to the substrate in a manner knownper se, for example by the method described in EP 0 139 557. The coatingof tin dioxide doped with antimony, arsenic, bismuth, copper, gallium orgermanium is applied to the platelet-shaped substrate in an amount ofabout 25-100%, in particular in an amount of about 50-75%.

Pigments which are particularly suitable for laser marking are thosebased on platelet-shaped metal oxides or platelet-shaped substrates,preferably mica, coated with one or more metal oxides. Particularlysuitable pigments are those which are distinguished by the fact that thebase substrate is first coated with an optionally hydrated silicondioxide coating before the doped tin dioxide coating is applied. Suchpigments are described in DE 38 42 330. In this case, the substrate issuspended in water and the solution of a soluble silicate is added at asuitable pH; if necessary, the pH is kept in the suitable range bysimultaneous addition of acid. The silicic acid-coated substrate can beseparated off from the suspension before the subsequent coating with thetin dioxide coating and worked up or coated directly with the doped tindioxide coating.

All known thermoplastics, as described, for example, in Ullmann, Vol.15, pp. 457 ff., published by VCH, can be used for laser marking.Examples of suitable plastics are polyethylene, polypropylene,polyamides, polyesters, polyester esters, polyether esters,polyphenylene ethers, polyacetal, polybutylene terephthalate, polymethylmethacrylate, polyvinyl acetal, polystyrene,acrylonitrile-butadiene-styrene (ABS), acrylonitrile-styrene-acrylate(ASA), polycarbonate, polyether sulphones, polyether ketones andcopolymers and/or mixtures thereof. Particularly suitable arethermoplastic polyurethanes (TPUs) owing to their good mechanicalproperties and the inexpensive processing methods. Thermoplasticpolyurethanes have long been known from numerous patents and otherpublications, for example from GB 1,057,018 and EP 0 564 931.

The pigments are incorporated into the thermoplastic by mixing theplastic granules with the pigment and then moulding the mixture atelevated temperature. If necessary, adhesives, organicpolymer-compatible solvents, stabilizers and/or surfactants which areheat-stable under the working conditions can be added to the plasticgranules during incorporation of the pigments. The plasticgranule/pigment mixture is generally prepared by introducing the plasticgranules into a suitable mixer, wetting the granules with any additivesand then adding and admixing the pigment. The pigmentation of plastic isgenerally carried out via a colour concentrate (masterbatch) orcompound. The resultant mixture can then be processed directly in anextruder or injection-moulding machine. The mouldings formed onprocessing exhibit very homogeneous distribution of the pigment. Thelaser marking is then carried out.

The invention also relates to a process for the preparation of the novellaser-markable plastics, characterized in that a thermoplastic is mixedwith the pigment and then moulded at elevated temperature.

The inscription with the laser is carried out by introducing the testspecimen into the ray path of a pulsed laser, preferably an Nd-YAGlaser. Inscription using an excimer laser is also possible, for exampleby means of a mask technique. However, the desired results can also beachieved using other conventional types of laser which have a wavelengthin a region of high absorption of the pigment used. The marking obtainedis determined by the irradiation time (or pulse number in the case ofpulsed lasers) and irradiation output of the laser and of the plasticsystem used. The output of the laser used depends on the particularapplication and can readily be determined in the individual case by aperson skilled in the art.

The novel pigmented plastic can be used in all areas where printingprocesses have hitherto been employed for the inscription of plastics.For example, mouldings of the novel plastic can be used in theelectrical, electronics and motor vehicle industries. The labelling andinscription of, for example, cables, wires, trim strips or functionalparts in the heating, ventilation and cooling sectors or switches,plugs, levers and handles comprising the novel plastic is possible evenat poorly accessible points with the aid of laser light. Owing to itslow heavy-metal content, the novel plastic system can furthermore beemployed in packaging in the foodstuffs sector or in the toys sector.The markings on packaging are distinguished by the fact that they arewipe- and scratch-resistant, stable during subsequent sterilizationprocesses, and can be applied in a hygienically pure manner during themarking process. Complete label motifs can be applied durably to thepackaging for a reusable system. A further important area of applicationfor laser inscription comprises plastic marks for the individual taggingof animals, known as cattle tags or ear marks. Via a bar-code system,the information specific to the animal is stored and can then berecalled again, when required, with the aid of a scanner. Theinscription must be very durable since the marks in some cases remain onthe animals for a number of years.

It is thus possible to laser-mark plastic articles or mouldingscomprising the novel plastic.

The examples below are intended to illustrate the invention, but withoutrepresenting a limitation. Percentages are by weight.

EXAMPLES Example 1

Yellow injection mouldings are produced from a thermoplasticpolyether-polyurethane having a Shore hardness of 85 A, based on 1000parts of polytetrahydrofuran having a molecular weight of 1000, 600parts of MDI (methylenediphenyl 4,4′-diisocyanate) and 126 parts. of1,4-butanediol, and contain 1% of the mica pigment Minatec® 31 CM (TiO₂mica pigment having an antimony-doped tin dioxide coating, commercialproduct from E. Merck,Darmstadt), 0.6% of titanium dioxide and 0.25% ofQuinophthalone Yellow (Paloithol® K 0691 from BASF). The pigments areadded to the polyether-TPU in the form of concentrates before theinjection-moulding operation. After the inscription by means of anNd-YAG laser at a current strength of 15 A and a writing speed of 400mm/s, the tablets exhibit a clear, abrasion-resistant inscription. Thereflection values, measured using a PSC quick-check 300 and a 670 nmreading pen, are 66% on the matrix and 21% in inscribed areas. A barcode is readily readable.

Mechanical properties Tensile Elongation Shore A strength/Mpa at break/%hardness Abrasion/mm³ DIN 53S04 DIN 53504 DIN 53505 DIN 53516 50 650 8530

Example 2

Orange injection mouldings are produced from a thermoplasticpolyether-polyurethane having a Shore hardness of 90 A, based on 1000parts of polytetrahydrofuran having a molecular weight of 1000, 700parts of MDI and 162 parts of 1,4-butanediol, and contain 1% of the micapigment Minatec® 30 CM (SiO₂-coated TiO₂ mica pigment having anantimony-doped tin dioxide coating commercial product from Merck,Darmstadt), 0.6% of titanium dioxide and 0.5% of lead chromate pigment(Krolor Yellow 787 D) and 0.1% of lead molybdate pigment (Krolor Orange789 D). The pigments are added to the polyether-TPU in the form ofconcentrates before the injection-moulding operation.

The inscription by means of an Nd-YAG laser at 13 amperes, a pulsefrequency of 5 kHz and at 600 mm/s is high in contrast andabrasion-resistant.

The reflection values using the PSC 300 are 56% on the matrix and 25% inthe inscribed areas.

Example 3

Orange injection mouldings are produced from a thermoplasticpolyether-polyurethane having a Shore hardness of 95 A, based on 1000parts of polytetrahydrofuran having a molecular weight of 1000, 830parts of MDI and 209 parts of 1,4-butanediol, and 2% of a UV stabilizerconcentrate. In addition, the tablets contain 1% of the mica pigmentMinatec® 31 CM, 0.6% of titanium dioxide and 0.25% of QuinophthaloneYellow and 0.02% of the azo pigment Paliotol Yellow K 2270. The pigmentsare added to the polyether-TPU in the form of concentrates before theinjection-moulding operation.

The inscription by means of an Nd-YAG laser at 18 amperes and 600 mm/sis high in contrast and abrasion-resistant.

The reflection values using the PSC 300 are 67% on the matrix and 24% inthe inscribed areas.

Example 4

White injection mouldings are produced from a polyether-polyurethanehaving a Shore hardness of 90 A, and contain 1% of the mica pigmentMinatec® 31 CM and 1% of titanium dioxide. The pigments are added to thepolyether TPU in the form of concentrates before the injection-mouldingoperation.

The inscription by means of an Nd-YAG laser at 14 amperes, a pulsefrequency of 5 kHz and at 400 mm/s is high in contrast andabrasion-resistant.

The reflection values using the PSC 300 are 68% on the matrix and 28% inthe inscribed areas.

Example 5

Yellow injection mouldings are produced from a polyether ester having aShore hardness of 42 D, based on polytetrahydrofuran and polybutylenetherephthalate [sic]. The tablets contain 1% of the mica pigment.Minatec® 31 CM, 0.6% of titanium dioxide and 0.25% of QuinophthaloneYellow. The pigments are added in the form of concentrates before theinjection-moulding operation.

A high-contrast inscription is applied by means of an Nd-YAG laser.

Example 6

Yellow injection mouldings are produced from a polyester ester having aShore hardness of 55 D, based on polybutylene therephthalate andpolycaprolactone. The tablets contain 1.5% of the mica pigment Minatec®31 CM, 0.6% of titanium dioxide and 0.25% of Quinophthalone Yellow. Thepigments are added in the form of concentrates before theinjection-moulding operation.

A high-contrast inscription is applied by means of an Nd-YAG laser.

Example 7

Orange injection mouldings are produced from a polyester-polyurethanehaving a Shore hardness of 90 A, based on 100 parts ofpoly(1,4-butanediol)-hexane-1,6-diol adipate having a molecular weightof 2000, 580 parts of MDI and 162 parts of 1,4-butanediol and contain1.5% of the mica pigment Minatec® 30 CM, 0.6% of titanium dioxide, 0.5%of lead chromate pigment (Krolor Yellow 787 D) and 0.1% of leadmolybdate pigment (Krolor Orange 789 D). The pigments are added to thepolyether-TPU in the form of concentrates before the injection-mouldingoperation.

The inscription by means of an Nd-YAG laser is durable and high incontrast.

Example 8

Plugs produced from a moulding composition comprising polyamide(Ultramid® A3K BASF, Ludwigshafen) and 0.3% of Minatec® 30 CM areprovided with a dark inscription on a white background. The inscriptionby means of an Nd-YAG laser gives clean and smooth motifs. The colourchange occurring close to the pigment is very clear in the polyamidewithout any significant change to the polymer.

Example 9

Light switches produced from a moulding composition comprisingpolyacetal (Hostaform® C902, from Hoechst) and 1% of Minatec® 31 CM areprovided with a grey marking on a white background by means of theNd-YAG laser.

Example 10

Films comprising polypropylene (PPH10 from DSM) and 1% of Minatec® 30 CMare provided with a sharp-edged marking by means of the Nd-YAG laser,the backing material exhibiting no foaming effect.

Comparative Example 1

Yellow injection mouldings are produced from a thermoplasticpolyether-polyurethane having a Shore hardness of 95 A, based on 1000parts of polytetrahydrofuran having a molecular weight of 1000, 830parts of MDI and 209 parts of 1,4-butanediol, and 2% of a UV stabilizerconcentrate. In addition, the tablets contain 0.6% of titanium dioxideand 0.25% of Quinophthalone Yellow and 0.02% of the azo pigment PaliotolYellow K 2270. The pigments are added to the polyether TPU in the formof concentrates before the injection-moulding operation.

The composition and colouring correspond to Example 3, but there is nolaser contrasting agent. Inscription by means of an Nd-YAG laser isimpossible.

Comparative Example 2

Yellow injection mouldings are produced from a thermoplasticpolyether-polyurethane having a Shore hardness of 95 A, based on 1000parts of polytetrahydrofuran having a molecular weight of 1000, 830parts of MDI and 209 parts of 1,4-butanediol, and 2% of a UV stabilizerconcentrate. In addition, the tablets contain 5% of antimony trioxide,0.6% of titanium dioxide and 0.25% of Quinophthalone and 0.02% of theazo pigment Paliothol® Yellow K 2270. The pigments are added to thepolyether PTU in the form of concentates before the injection-mouldingoperation.

The composition and colouring correspond to Example 3, apart from thelaser contrasting agent, and to Comparative Example 1.

The inscription is carried out by means of an Nd-YAG laser at 18 amperesand 600 mm/s.

The reflection values using the PSC 300 are 69% on the matrix and 22% inthe inscribed areas.

What is claimed is:
 1. A laser-markable plastic which comprises athermoplastic containing a pigment having a platelet-shaped substratewhich has been coated with an optionally hydrated silicon dioxidecoating or a coating of another insoluble silicate and over that coatinganother coating of tin dioxide doped with 0.5-50% by weight of antimony,arsenic, bismuth, copper, gallium, germanium, or a corresponding oxidethereof.
 2. The laser-markable plastic of claim 1, wherein theproportion of the pigment is 0.1-3% by weight based on the plastic.
 3. Alaser-markable plastic according to claim 1, wherein the platelet-shapedsubstrate is mica platelets, SiO₂ flakes or mica platelets coated withone or more metal oxides.
 4. A laser-markable plastic according to claim1, wherein the proportion of pigment is 0.1-2% by weight, based on theplastic.
 5. A laser-markable plastic according to claim 1, wherein thetin dioxide coating of the pigment is doped with 0.5-50% by weight ofantimony, or a corresponding oxide thereof.
 6. A laser-markable plasticaccording to claim 1, wherein the thermoplastic is a thermoplasticpolyurethane.
 7. A laser-markable plastic according to claim 1, whereinthe thermoplastic is a polyether ester or polyester ester.
 8. Alaser-markable plastic according to claim 1, which additionallycomprises a colored pigment.
 9. A plastic molding comprising alaser-markable plastic according to claim
 1. 10. A method for making amolding laser-markable which comprises incorporating in the molding alaser-markable plastic according to claim
 1. 11. A laser-markableplastic according to claim 1, which is markable by a Nd-YAG laser.
 12. Aprocess for the preparation of a laser-markable plastic, whichcomprises: mixing thermoplastic granules with a pigment having aplatelet-shaped substrate which has been coated with an optionallyhydrated silicon dioxide coating or a coating of another insolublesilicate and over that coating another coating of tin dioxide doped with0.5-50% by weight of antimony, arsenic, bismuth, copper, gallium,germanium, or a corresponding oxide thereof, and then molding atelevated temperature.
 13. A pigment having a platelet-shaped substratewhich has been coated with an optionally hydrated silicon dioxidecoating or a coating of another insoluble silicate and over that coatinganother coating of tin dioxide doped with 0.5-50% by weight of antimony,arsenic, bismuth, copper, gallium, germanium, or a corresponding oxidethereof.
 14. The pigment according to claim 13, wherein theplatelet-shaped substrate is mica platelets, SiO₂ flakes or micaplatelets coated with one or more metal oxides.
 15. The pigmentaccording to claim 13, wherein the tin dioxide coating of the pigment isdoped with 0.5-50% by weight of antimony, or a corresponding oxidethereof.
 16. The laser-markable plastic of claim 1, wherein the tindioxide coating of the pigment is doped with 0.5-20% by weight ofantimony, arsenic, bismuth, copper, gallium, germanium, or acorresponding oxide thereof.
 17. The process of claim 12, wherein thetin dioxide coating of the pigment is doped with 0.5-20% by weight ofantimony, arsenic, bismuth, copper, gallium, germanium, or acorresponding oxide thereof.
 18. The pigment of claim 13, wherein thetin dioxide coating of the pigment is doped with 0.5-20% by weight ofantimony, arsenic, bismuth, copper, gallium, germanium, or acorresponding oxide thereof.